Sonic logging tool including receiver and spacer structure

Abstract

A formation logging tool having a substantially continuous central mandrel with regularly spaced mass blocks disposed thereon, at least some of the mass blocks carrying sensors such as receivers. By adopting this structure, the tool can be made to behave as a mass-spring structure and its flexural and extensional behaviour controlled such that its dispersion curve does not extend into the dispersion curve of the formation to be logged. The structure can be applied to the whole of the logging tool or just to the receiver section and/or any spacer section between the receiver and the transmitter section.

1. An acoustic borehole logging tool comprising:
 at least one acoustic signal source; 
 at least one acoustic receiver section; and 
 a spacer, disposed between the acoustic signal source and the receiver section, comprising a substantially continuous mandrel having a series of mass blocks, each mass block having an inner surface defining a cavity, a region of the inner surface of the mass blocks being in direct contact with the outer surface of the mandrel such that the mass blocks are fixedly secured on the mandrel, the mandrel and mass blocks secured thereon being configured, and the mass blocks being positioned on the mandrel, so that the spacer is configured to behave acoustically as a mass-spring structure. 
 
   
   
     2. A logging tool as claimed in  claim 1 , comprising a tool body including a substantially continuous structural member extending between the acoustic signal source and the acoustic receiver section. 
   
   
     3. A logging tool as claimed in  claim 1 , comprising monopole and dipole acoustic signal sources. 
   
   
     4. A logging tool as claimed in  claim 1 , wherein the mandrel is hollow and defines a conduit for wiring between sections of the logging tool disposed on either side of the spacer. 
   
   
     5. A logging tool as claimed in  claim 1 , where no outer sleeve is provided for the series of blocks. 
   
   
     6. A logging tool as claimed in  claim 1 , wherein each block has an opening defined therein to allow cleaning of a region around the mandrel. 
   
   
     7. A logging tool as claimed in  claim 1 , wherein the receiver section includes mass blocks. 
   
   
     8. A logging tool as claimed in  claim 7 , wherein at least some of the mass blocks carry acoustic receiver elements. 
   
   
     9. A logging tool as claimed in  claim 8 , wherein receiver elements are disposed on each of a number of neighboring mass blocks so as to form a linear array. 
   
   
     10. A logging tool as claimed in  claim 9 , wherein the array is formed from at least eight adjacent mass blocks. 
   
   
     11. A logging tool as claimed in  claim 8 , wherein each mass block carrying acoustic receiver elements has a number of elements disposed around the periphery thereof. 
   
   
     12. A logging tool as claimed in  claim 8 , wherein each receiver element has associated electronic circuitry to provide a digital output therefrom, such that the output from each receiver element is converted into digital format by circuitry located adjacent to its respective receiver. 
   
   
     13. A logging tool as claimed in  claim 12 , wherein the electronics are located around the central mandrel adjacent associated receiver elements. 
   
   
     14. A logging tool as claimed in  claim 7 , wherein at least one mass block carries at least four receiver elements. 
   
   
     15. An acoustic borehole logging tool comprising:
 at least one acoustic signal source; 
 an acoustic receiver sonde comprising a substantially continuous mandrel having a series of mass blocks, each mass block having an inner surface defining a cavity, a region of the inner surface of the mass blocks being in direct contact with the outer surface of the mandrel such that the mass blocks are fixedly secured on the mandrel, at least some of the mass blocks carrying acoustic receiver elements, the mass blocks and the mandrel being configured and the mass blocks being positioned on the mandrel to provide structural support and behave acoustically as a mass-spring structure. 
 
   
   
     16. A logging tool as claimed in  claim 15 , wherein receiver elements are disposed on each of a number of neighboring mass blocks so as to form a linear array. 
   
   
     17. A logging tool as claimed in  claim 16 , wherein the array is formed from at least eight adjacent mass blocks. 
   
   
     18. A logging tool as claimed in  claim 15 , wherein each mass block carrying acoustic receiver elements has a number of elements disposed around the periphery thereof. 
   
   
     19. A logging tool as claimed in  claim 15 , wherein each receiver element has associated electronic circuitry to provide a digital output therefrom, such that the output from each receiver element is converted into digital format by circuitry located adjacent to its respective receiver. 
   
   
     20. A logging tool as claimed in  claim 19 , wherein the electronics are located around the central mandrel adjacent associated receiver elements. 
   
   
     21. A logging tool as claimed in  claim 15 , wherein electronics are located around the mandrel adjacent associated receiver elements. 
   
   
     22. A logging tool as claimed in  claim 15 , wherein at least one mass block carries at least four receiver elements. 
   
   
     23. A logging tool as claimed in  claim 15 , comprising at least one receiver element mounted on at least one mass block. 
   
   
     24. A logging tool as claimed in  claim 23 , comprising a piezoelectric cylinder. 
   
   
     25. A logging tool as claimed in  claim 23 , comprising a stack of piezoelectric plates. 
   
   
     26. A logging tool as claimed in  claim 23 , wherein one or more mass block carrying at least one receiver element has a plurality of elements around the periphery thereof. 
   
   
     27. A logging tool as claimed in  claim 26 , wherein a mass block carries at least four receiver elements. 
   
   
     28. A logging tool as claimed in  claim 23 , wherein a number of acoustic receiver elements are carried on multiple mass blocks so as to form an array. 
   
   
     29. A logging tool as claimed in  claim 15 , comprising electronics disposed on one or more mass blocks. 
   
   
     30. A logging tool as claimed in  claim 29 , wherein the electronics includes receiver front end electronics. 
   
   
     31. A logging tool as claimed in  claim 30 , wherein the electronics are located adjacent associated receiver elements. 
   
   
     32. An acoustic borehole logging tool comprising:
 at least one acoustic signal source; 
 a receiver section; and 
 a spacer section disposed between the acoustic signal source and the receiver section; wherein the receiver section and the spacer section each comprise a substantially continuous mandrel having a series of mass blocks, each mass block having an inner surface defining a cavity, a region of the inner surface of the mass blocks being in direct contact with the outer surface of the mandrel such that the mass blocks are fixedly secured on the mandrel, the mandrel and mass blocks secured thereon being configured and the mass blocks being positioned on the mandrel to provide structural support and behave acoustically as a mass-spring structure. 
 
   
   
     33. A logging tool as claimed in  claim 32 , wherein receiver elements are disposed on a number of neighboring mass blocks so as to form a linear array and each receiver element has associated electronic circuitry to provide a digital output therefrom, such that the output from each receiver element is converted into digital format by circuitry located adjacent to its respective receiver.